Created attachment 8458071 [details] Not-so-minmal test case with fixed styles
Odd. The case with inlined styles does not work to reproduce the issue (i.e. its behaviour is correct). Please download attachment #8458063 [details] instead.
Created attachment 8458698 [details] Minimal test case I've created a minimal test case that reproduces the problem. Still looking into why it happens.
Forgot to repeat the STR: 1. Open attachment 8458698 [details] in a tab and open the debugger. 2. Go to the events pane and click to break on mouseout events. 3. Move the mouse cursor over the button and hit F8 to pause. 4. Observe that leaving the content window will run the event handler and not pause execution (although I've got a pause at one time).
I suspect that this is related to bugs 801304 and 1044074. The gist of those bugs is that nested event loops don't properly suppress all events. Something similar might be going on here.
No time for this in the near future unfortunately.